实时数据。无法分配给“值”：setter 受保护/受保护并打包/用于合成扩展

Question

我正在尝试按照 android documentation 中所述使用 LiveData 实现数据库观察器。

只要我在 Kotlin 中编程，我就会调整函数（最初用 Java 编写）。

在尝试保存数据时，我发现了这个问题。

Cannot assign to ‘value’: the setter is protected/*protected and package*/ for synthetic extension in ‘<library Grade: android.arch.livecycle:livedata-core-1.1.1>’

有人遇到过这个问题吗？

这是我的代码：

ViewModel:

class ProfileViewModel: ViewModel() {

    object FirstName: MutableLiveData<String>()

    fun getCurrentName(): LiveData<String> {
        return FirstName
    }
}

Fragment

class ProfileFragment{

    private lateinit var model: ProfileViewModel

    // this is called onViewCreated. inputFirstName is an Edittext.
    override fun setUp() {
        model = ViewModelProviders.of(this).get(ProfileViewModel::class.java)

        val nameObserver = Observer<String> { firstName ->
            inputFirstName.text = SpannableStringBuilder(firstName)
        }

        model.getCurrentName().observe(this, nameObserver)
    }

    fun saveProfileData() {
        val firstName = inputFirstName.text.toString()
        model.getCurrentName().value = firstName
    }
}

Answer 1

正如@spkink 建议的那样：

替换

fun getCurrentName(): LiveData<String>

和

fun getCurrentName(): MutableLiveData<String>

错误是因为setValue(T value)在LiveData中是protected（所以不能调用）而在MutableLiveData中是public。

Answer 2

model.getCurrentName().value = firstName

请考虑在 ViewModel 中处理名称值更新可能是一个更清晰的解决方案，因为您避免了可变的 public 属性。此外，这里可能会导致更糟糕的效果，因为您有两个可变的 public 实时数据属性。例如：

ViewModel

// keep your MutableLiveData private, so it's easier to find where mutations might happen in your code
// I changed a little how the variable is declared, as the way below seems to more common nowadays
private var _firstName = MutableLiveData<String>()

// LiveData should remain public, being a read-only interface with other classes like your fragment
fun getCurrentName(): LiveData<String> {
    return _firstName
}

/*
An alternative 
val firstName = LiveData<String>
    get() = _firstName
*/

fun setFirstName(firstName: String){
    _firstName.value = firstName
}

Fragment

fun saveProfileData() {
    val firstName = inputFirstName.text.toString()
    // the value update will be handled inside the ViewModel, where the private mutable live data property is located
    model.setFirstName(firstName)
}

实时数据。无法分配给“值”：setter 受保护/受保护并打包/用于合成扩展

LiveData. Cannot assign to ‘value’: the setter is protected/protected and package/ for synthetic extension

android

kotlin

android-livedata

android-architecture-components

实时数据。无法分配给“值”：setter 受保护/*受保护并打包*/用于合成扩展

LiveData. Cannot assign to ‘value’: the setter is protected/*protected and package*/ for synthetic extension

android

kotlin

android-livedata

android-architecture-components

实时数据。无法分配给“值”：setter 受保护/受保护并打包/用于合成扩展

LiveData. Cannot assign to ‘value’: the setter is protected/protected and package/ for synthetic extension